WL#1798: Make minimum keyblock settable as variable

Currently minimum key block size (1K) is hardcoded while it should be settable
as server startup variable.
The task WL#602 is extending this task by allowing value setting on Index and
Table level.
Setting minumum key block size is very useful in 2 reasons
1) To make LOAD INDEX work which currently does not work if indexes have different
block size (workaround)
2) To have IO in OS_PAGE size blocks which are usually 4K.
According to the Igor this task should be rather easy to implement.